Caesium-137

Caesium-137 (137
55Cs
, Cs-137), cesium-137, or radiocaesium, is a radioactive isotope of caesium which is formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nuclear weapons. It is among the most problematic of the short-to-medium-lifetime fission products because it easily moves and spreads in nature due to the high water solubility of caesium's most common chemical compounds, which are salts.

Caesium-137 has a half-life of about 30.17 years. About 95 percent decays by beta emission to a metastable nuclear isomer of barium: barium-137m (^137mBa, Ba-137m). The remainder directly populates the ground state of barium-137, which is stable. Ba-137m has a half-life of about 153 seconds, and is responsible for all of the emissions of gamma rays in samples of caesium-137. One gram of caesium-137 has an activity of 3.215 terabecquerel (TBq).

The main photon peak of Ba-137m is 662 keV.

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